Synthesis of human fetal and adult hemoglobins is being studied in a plasma clot system in the presence of erythropoietin. Erythroid precursors are obtained from post-mortem specimens of fetal liver, umbilical cord blood and adult peripheral blood by Ficoll-Hypaque centrifugation and separation of mononuclear cells. Erythroid colonies which develop are exposed to radioactive amino acid and globin chain synthesis is measured. The object of these studies is to explore the differential control of human fetal and adult hemoglobin production.